Looking to plot yaw motion of an aircraft but get "Array indices must be positive integers or logical values." Error

clear all

clc

N_phidot = -0.36752;

N_phi = -8.3569;

N_delta_r = -3.8313;

delta_R = -10;

sigma = 1/2.*N_phi;

omega_d = (-N_phi-1/4.*(-N_phidot)^2)^(1/2);

xi = 1/2.*(-N_phidot/-N_phi^(1/2));

omega_n = (-N_phi)^(1/2);

beta = (N_delta_r/N_phi).*delta_R;

phi = atan(sigma/omega_d);

alpha = -1/2.*beta.*(omega_n/omega_d);

Tau = 2*3.14/omega_d;

xiomega_n = 1/Tau;

t = [0:0.01:10];

Phi(t) = beta.*(1-(omega_n/omega_d.*exp(-sigma.*t).*cos((omega_d.*t)-phi)));

Phidot(t) = diff(Phi(t));

figure(1)

plot(t,phidot)

title('Phidot vs time')

xlabel('seconds')

ylabel('degrees')

figure(2)

plot(t, phi)

title('Phi vs time')

xlabel('seconds')

ylabel('degrees')

Error Messages

Array indices must be positive integers or logical values.

Error in HW1MAE503Q2 (line 22)

Phi(t) = beta.*(1-(omega_n/omega_d.*exp(-sigma.*t).*cos((omega_d.*t)-phi)));

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